Since fiber reinforced plastic composite material (hereinafter, referred to as “FRP”) is light in weight and high in strength and rigidity, the FRP is widely used in sports and leisure use or industrial use including automobiles or airplanes.
The FRP is also used in the casings of an electric/electronic device such as a personal computer, a household electrical appliance, and a medical instrument. In the electric/electronic device such as a personal computer and a phone, there is a need to reduce the size, weight, and thickness of the component constituting the device due to the mobile trend therefor. Particularly, in the case that, when a load is applied to the casing of such a device from the outside thereof, a part of the casing is bent to thereby and come into contact with the inner component, the inner component or the casing may be damaged. For this reason, there is a need to increase the strength and rigidity of the casing in order to prevent such problems.
Patent Document 1 discloses a sandwich structure (III) including a core (I) that forms at least one structure of a honeycomb structure, an island structure, and a structure having a void penetrating the structure in a direction parallel to the surface thereof and a fiber reinforced material (II) that is disposed on both surfaces of the core (I) and is formed of continuous reinforcing fibers and a matrix resin. Although it is described that the structure is useful as the structure that needs to be excellent in the light weight property, the thin thickness property, and the high-volume production capability, the light weight property, the thin thickness property, and the high-volume production capability are not obtained as expected due to the arrangement of the fiber reinforced material (II) on both surfaces of the core (I).
Further, Patent Document 1 discloses a configuration in which outsert-injection-molding is performed by forming a thermoplastic resin layer on the outermost layer of the sandwich structure (III) in order to form a resin structure (a thermoplastic resin composition) as a frame with hinges, bosses, and/or ribs on the outer periphery of the sandwich structure (III). Patent Document 1 discloses an advantage that a portion provided with thermoplastic resin layer is bonded by the overlap length of about 5 mm so as to be strongly integrated. However, in the structure disclosed in Patent Document 1, a bonding portion between the sandwich structure (III) and the resin structure is located on the upper surface of the resin structure. That is, since the bonding portion is formed by overlapping the sandwich structure (III) and the resin structure in the up and down direction, the thickness of the sandwich structure (III) at the overlapping bonding portion impedes the reduction of the thickness of the casing.
Further, since the sandwich structure (III) is located on the upper surface of the resin structure, the lateral end surface of the sandwich structure (III) is formed on the surface of the product. As a result, the quality of the product appearance is obtained only to a limited degree.
Patent Document 2 discloses a composite molded article formed of a plate member and a resin structure, wherein the plate member includes surface layer bases that are located at each of the topside and underside surfaces and a core layer base that is located between the both surface layer bases. Patent Document 2 discloses a composite molded article in which each surface layer base is formed of a fiber reinforced resin and the core layer base is formed of a soft material softer than the fiber reinforced resin forming each surface layer base. In Patent Document 2, it is described that the composite molded article is useful as a composite molded article that needs to be excellent in the light weight property and the thin thickness property. However, placing the core layer base which is formed of a soft material between the surface layer bases impede the light weight property and the thin thickness property.
Further, the composite moldeded article disclosed in Patent Document 2 includes a bonding surface in which the plate member and the resin structure are bonded to each other at the facing end surfaces, at least a part of the bonding interface is an undulating bonding interface in which the side end faces of the surface layer bases and the side end face of the resin structure are bonded to each other while having an undulating shape, and the front end of the resin structure in the undulating bonding interface is penetrating between both surface layer bases so that the front end is formed as a resin structure penetrating front end. Patent Document 2 describes that such a bonding surface may obtain a sufficient bonding strength. In the case where the flowability of the resin is low or the injection pressure may not be increased during the injection-molding, the convex shape of the resin structure is not sufficiently inserted into the core layer base, and hence the sufficient bonding strength between the plate member and the resin structure is hard to obtain.